Conventionally, in order to detect a pressure in a gas pipe for supplying a gas to a gas engine, a pressure sensor is provided on the gas pipe. An example of a gas pressure detection device is shown in FIG. 2 of Patent Document 1. On the gas pipe, a pressure sensor, gas valves for opening and closing the gas pipe, and a gas engine are disposed in order. The gas valves consist of two electromagnetic valves and a pressure adjustment valve. When the gas valve opens the gas pipe, the pressure in the gas pipe changes. The pressure change in the gas pipe is detected by the pressure sensor.
The detection device of Patent Document 1 always monitors an output signal of a pressure switch, whereby when the gas pressure in the gas pipe is reduced, the detection device shuts down the gas engine. The reason for this is as follows: If operation of the gas engine is continued in a state in which the gas pressure is reduced, the pressure in the gas pipe becomes a negative pressure, so that there is fear of the occurrence of failure in which air is mixed in the gas pipe from an opening formed in the gas pipe, for example, from a gas supply port to other gas equipment. In the detection device of Patent Document 1, the reduction in the gas pressure is detected, whereby the occurrence of such failure can be prevented.
Incidentally, for power saving, a pressure switch having a b-contact is widely used as the pressure sensor in Japan. When the gas pressure is higher than a set pressure, the b-contact is open, which becomes a non-conductive state. On the other hand, when the gas pressure is lower than the set pressure, the b-contact is closed, which becomes a conductive state. That is, when the gas pressure is lower than the set pressure, a detection signal is outputted from the pressure switch, and when the gas pressure is higher than the set pressure, the signal is not outputted from the pressure switch. Namely, since the pressure switch becomes the conductive state only at the occurrence of an abnormality in which the gas pressure in the gas pipe is reduced more than the set pressure, power consumption of the pressure switch is kept low. In particular, if a battery is used as a power supply for the pressure switch instead of a system power supply, the battery life can be extended.